Sulfur vulcanizable rubber containing sodium thiosulfate pentahydrate

ABSTRACT

A method is disclosed for increasing the rate of vulcanization of a sulfur rubber composition comprising heating a sulfur vulcanizable rubber composition to a temperature ranging from 100° C. to 200° C., said rubber composition, containing a sulfur vulcanizable rubber, a sulfenamide compound and a hydrated thiosulfate. Addition of the hydrated thiosulfate to a sulfur vulcanizable rubber and a sulfenamide compound significantly increases the rate of vulcanization of the rubber.

FIELD OF THE INVENTION

The present invention relates to increasing the rate of vulcanization ofa sulfur curable rubber composition.

BACKGROUND OF THE INVENTION

The "rate of cure" is defined as the rate at which crosslinking and thedevelopment of the stiffness (modulus) of a rubber compound occurs. Asthe rubber compound is heated, the properties of the rubber compoundchange from a soft plastic to a tough elastic material. During thecuring step, crosslinks are introduced, which connect the long polymerchains of the rubber. As more crosslinks are introduced, the polymerchains become more firmly connected and the stiffness or modulus of thecompound increases. The rate of cure is an important vulcanizationparameter since it in part determines the time the compound must becured, i.e., the "cure time". In the manufacture of vulcanized rubberarticles, significant cost savings can be realized through a reductionof cure time. Through enhanced rates of cure, the cure time required tomeet minimum states of cure can be reduced. Given the above, extensiveresearch has been conducted in order to shorten the cure times ofrubbers. Therefore, there exists a need for improved methods whichenhance the rate of cure in the absence of imparting undesirableproperties to the vulcanizate.

SUMMARY OF THE INVENTION

The present invention relates to the use of a sulfenamide compound and ahydrated thiosulfate in a sulfur vulcanizable rubber.

DETAILED DESCRIPTION OF THE INVENTION

There is disclosed a method for increasing the rate of vulcanization ofa sulfur vulcanizable rubber composition by heating a sulfurvulcanizable composition to a temperature ranging from 100° C. to 200°C., said rubber composition comprising

(a) a sulfur vulcanizable rubber

(b) from 0.5 phr to 5 phr of a sulfenamide compound of the generalformula: ##STR1## wherein R¹ is hydrogen, an acyclic aliphatic grouphaving from about 1 to 10 carbon atoms, or a cyclic aliphatic grouphaving from about 5 to 10 carbon atoms; and R² is hydrogen, a cyclicaliphatic group having from 5 to 10 carbon atoms or amercaptobenzothiazolyl group of the formula: ##STR2##

(c) from 0.05 to 10 phr of a hydrated thiosulfate.

There is also disclosed a sulfur vulcanizable composition comprising asulfur vulcanizable rubber composition comprising

(a) a sulfur vulcanizable rubber

(b) from 0.5 phr to 5 phr of a sulfenamide compound of the generalformula: ##STR3## wherein R¹ is selected from the group consisting ofhydrogen, acyclic aliphatic groups having from about 1 to 10 carbonatoms, and cyclic aliphatic groups having from about 5 to 10 carbonatoms; and R² is selected from the group consisting of cyclic aliphaticgroups having from about 5 to 10 carbon atoms or amercaptobenzothiazolyl group of the formula: ##STR4##

(c) from 0.05 to 10 phr of a hydrated thiosulfate.

The present invention may be used to vulcanize sulfur vulcanizablerubbers or elastomers containing olefinic unsaturation. The phrase"rubber or elastomer containing olefinic unsaturation" is intended toinclude both natural rubber and its various raw and reclaim forms aswell as various synthetic rubbers. In the description of this invention,the terms "rubber" and "elastomer" may be used interchangeably, unlessotherwise prescribed. The terms "rubber composition", "compoundedrubber" and "rubber compound" are used interchangeably to refer torubber which has been blended or mixed with various ingredients andmaterials and such terms are well known to those having skill in therubber mixing or rubber compounding art. Representative syntheticpolymers are the homopolymerization products of butadiene and itshomologues and derivatives, for example, methylbutadiene,dimethylbutadiene and pentadiene as well as copolymers such as thoseformed from butadiene or its homologues or derivatives with otherunsaturated monomers. Among the latter are acetylenes, for example,vinyl acetylene; olefins, for example, isobutylene, which copolymerizeswith isoprene to form butyl rubber; vinyl compounds, for example,acrylic acid, acrylonitrile (which polymerize with butadiene to formNBR), methacrylic acid and styrene, the latter compound polymerizingwith butadiene to form SBR, as well as vinyl esters and variousunsaturated aldehydes, ketones and ethers, e.g., acrolein, methylisopropenyl ketone and vinylethyl ether. Specific examples of syntheticrubbers include neoprene (polychloroprene), polybutadiene (includingcis-1,4-polybutadiene), styrene-butadiene copolymers, polyisoprene(including cis-1,4-polyisoprene), butyl rubber, styrene-isoprenecopolymers, styrene-isoprene-butadiene terpolymers, methylmethacrylate-butadiene copolymers, methyl methacrylate-isoprenecopolymers, as well as ethylene/propylene terpolymers, also known asethylene/propylene/diene monomer (EPDM), and in particular,ethylene/propylene/dicyclopentadiene terpolymers. Mixtures of the aboverubber may be used. The preferred rubber or elastomers arestyrene/butadiene copolymer, polybutadiene, natural rubber andpolyisoprene.

The term "phr" as used herein, and according to conventional practice,refers to "parts by weight of a respective material per 100 parts byweight of rubber, or elastomer".

The first essential component of the present invention is the hydratedthiosulfate. The hydrated thiosulfate that is used may vary.Representative examples of such hydrated thiosulfates include BaS₂ O₃.H₂O, K₂ S₂ O₃.1.5 H₂ O, CaS₂ O₃.6H₂ O, MgS₂ O₃.6H₂ O, NiS₂ O₃.6H₂ O, CoS₂O₃.6H₂ O, SrS₂ O₃.5H₂ O, Na₂ S₂ O₃.5H₂ O, MnS₂ O₃.5H₂ O, Li₂ S₂ O₃.3H₂ Oand CdS₂ O₃.2H₂ O. Preferably, the hydrated thiosulfate is Na₂ S₂ O₃.5H₂O.

The hydrated thiosulfate used in the present invention may be added tothe rubber by any conventional technique such as on a mill or in aBanbury. The amount of hydrated thiosulfate may vary widely depending onthe type of rubber and other compounds present in the vulcanizablecomposition. Generally, the amount of hydrated thiosulfate is used in arange of from about 0.05 to about 10.0 phr with a range of 0.1 to about5.0 phr being preferred.

For ease in handling, the sodium thiosulfate pentahydrate salt may beused per se or may be deposited on suitable carriers. Examples ofcarriers which may be used in the present invention include silica,carbon black, alumina, kieselguhr, silica gel and calcium silicate.

The above sulfenamide compound is the second essential component of thepresent invention. The sulfenamide is generally present in an amount offrom about 0.5 to about 5 phr. Preferably, the sulfenamide is present inan amount ranging from about 0.70 to about 2.0 phr.

Representative of the sulfenamide compounds which may be used in thepresent invention include N-cyclohexyl-2-benzothiazylsulfenamide,N-t-butyl-2-benzothiazylsulfenamide,N,N-dicyclohexyl-2-benzothiazylsulfenamide,N-isopropyl-2-benzothiazylsulfenamide andN-t-butylbis-(2-benzothiazylsulfen)amide. Preferably, the sulfenamidecompound is N-cyclohexyl-2-benzothiazylsulfenamide.

The processing of the sulfur vulcanizable rubber is conducted in thepresence of a sulfur vulcanizing agent. Examples of suitable sulfurvulcanizing agents include elemental sulfur (free sulfur), an aminedisulfide, polymeric polysulfide or sulfur olefin adducts. Preferably,the sulfur vulcanizing agent is elemental sulfur. The sulfur vulcanizingagent may be used in an amount ranging from 0.5 to 8 phr, with a rangeof from 1.5 to 5.0 being preferred.

It is readily understood by those having skill in the art that therubber composition would be compounded by methods generally known in therubber compounding art, such as mixing the various sulfur-vulcanizableconstituent rubbers with various commonly used additive materials suchas, for example, curing aids, such as activators and retarders andprocessing additives, such as oils, resins including tackifying resinsand plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes,antioxidants and antiozonants and peptizing agents. As known to thoseskilled in the art, depending on the intended use of the sulfurvulcanizable and sulfur vulcanized material (rubbers), the additivesmentioned above are selected and commonly used in conventional amounts.Typical amounts of reinforcing type carbon blacks(s), for thisinvention, if used, are hereinbefore set forth. Typical amounts oftackifier resins, if used, comprise about 0.5 to about 10 phr, usuallyabout 1 to about 5 phr. Typical amounts of processing aids compriseabout 1 to about 50 phr. Such processing aids can include, for example,aromatic, napthenic, and/or paraffinic processing oils. Typical amountsof antioxidants comprise about 1 to about 5 phr. Representativeantioxidants may be, for example, diphenyl-p-phenylenediamine andothers, such as, for example, those disclosed in the Vanderbilt RubberHandbook (1978), pages 344-346. Typical amounts of antiozonants compriseabout 1 to 5 phr. Typical amounts of fatty acids, if used, which caninclude stearic acid comprise about 0.5 to about 3 phr. Typical amountsof zinc oxide comprise about 2 to about 5 phr. Typical amounts of waxescomprise about 1 to about 5 phr. Often microcrystalline waxes are used.Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typicalpeptizers may be, for example, pentachlorothiophenol anddibenzamidodiphenyl disulfide.

In one aspect of the present invention, the sulfur vulcanizable rubbercomposition is then sulfur-cured or vulcanized.

Vulcanization of the rubber composition of the present invention isgenerally carried out at conventional temperatures ranging from about100° C. to 200° C. Preferably, the vulcanization is conducted attemperatures ranging from about 110° C. to 180° C. Any of the usualvulcanization processes may be used such as heating in a press or mold,heating with superheated steam or hot air or in a salt bath.

In addition to the sulfenamide compounds, additional accelerators areused to control the time and/or temperature required for vulcanizationand to improve the properties of the vulcanizate. In one embodiment,only the sulfenamide may be used, i.e., primary accelerator. In anotherembodiment, combinations of a sulfenamide and a secondary acceleratormight be used with the secondary accelerator being used in smalleramounts (of about 0.05 to about 3 phr) in order to activate and toimprove the properties of the vulcanizate. Combinations of theseaccelerators might be expected to produce a synergistic effect on thefinal properties and are somewhat better than those produced by use ofeither accelerator alone. In addition, delayed action accelerators maybe used which are not affected by normal processing temperatures butproduce a satisfactory cure at ordinary vulcanization temperatures.Vulcanization retarders might also be used. Suitable types ofaccelerators other than the sulfenamides that may be used in the presentinvention are amines, disulfides, guanidines, thioureas, thiazoles,thiurams, dithiocarbamates and xanthates. If a second accelerator isused, the secondary accelerator is preferably a guanidine,dithiocarbamate or thiuram compound.

The rubber compositions of the present invention may contain sulfurcontaining organosilicon compounds.

Examples of suitable sulfur containing organosilicon compounds are ofthe formula:

    Z--Alk--S.sub.n --Alk--Z

in which Z is selected from the group consisting of ##STR5## where R¹ isan alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl;

R² is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbonatoms;

Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and n is aninteger of 2 to 8.

Specific examples of sulfur containing organosilicon compounds which maybe used in accordance with the present invention include:3,3'-bis(trimethoxysilylpropyl) disulfide,3,3'-bis(triethoxysilylpropyl) tetrasulfide,3,3'-bis(triethoxysilylpropyl) octasulfide,3,3'-bis(trimethoxysilylpropyl) tetrasulfide,2,2'-bis(triethoxysilylethyl) tetrasulfide,3,3'-bis(trimethoxysilylpropyl) trisulfide,3,3'-bis(triethoxysilylpropyl) trisulfide,3,3'-bis(tributoxysilylpropyl) disulfide,3,3'-bis(trimethoxysilylpropyl) hexasulfide,3,3'-bis(trimethoxysilylpropyl) octasulfide,3,3'-bis(trioctoxysilylpropyl) tetrasulfide,3,3'-bis(trihexoxysilylpropyl) disulfide,3,3'-bis(tri-2"-ethylhexoxysilylpropyl) trisulfide,3,3'-bis(triisooctoxysilylpropyl) tetrasulfide,3,3'-bis(tri-t-butoxysilylpropyl) disulfide, 2,2'-bis(methoxy diethoxysilyl ethyl) tetrasulfide, 2,2'-bis(tripropoxysilylethyl) pentasulfide,3,3'-bis(tricyclonexoxysilylpropyl) tetrasulfide,3,3'-bis(tricyclopentoxysilylpropyl) trisulfide,2,2'-bis(tri-2"-methylcyclohexoxysilylethyl) tetrasulfide,bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl3'-diethoxybutoxy-silylpropyltetrasulfide, 2,2'-bis(dimethylmethoxysilylethyl) disulfide, 2,2'-bis(dimethyl sec.butoxysilylethyl)trisulfide, 3,3'-bis(methyl butylethoxysilylpropyl) tetrasulfide,3,3'-bis(di t-butylmethoxysilylpropyl) tetrasulfide, 2,2'-bis(phenylmethyl methoxysilylethyl) trisulfide, 3,3'-bis(diphenylisopropoxysilylpropyl) tetrasulfide, 3,3'-bis(diphenylcyclohexoxysilylpropyl) disulfide, 3,3'-bis(dimethylethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis(methyldimethoxysilylethyl) trisulfide, 2,2'-bis(methylethoxypropoxysilylethyl) tetrasulfide, 3,3'-bis(diethylmethoxysilylpropyl) tetrasulfide, 3,3'-bis(ethyl di-sec.butoxysilylpropyl) disulfide, 3,3'-bis(propyl diethoxysilylpropyl)disulfide, 3,3'-bis(butyl dimethoxysilylpropyl) trisulfide,3,3'-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenylethoxybutoxysilyl 3'-trimethoxysilylpropyl tetrasulfide,4,4'-bis(trimethoxysilylbutyl) tetrasulfide,6,6'-bis(triethoxysilylhexyl) tetrasulfide,12,12'-bis(triisopropoxysilyl dodecyl) disulfide,18,18'-bis(trimethoxysilyloctadecyl) tetrasulfide,18,18'-bis(tripropoxysilyloctadecenyl) tetrasulfide,4,4'-bis(trimethoxysilyl-buten-2-yl) tetrasulfide,4,4'-bis(trimethoxysilylcyclohexylene) tetrasulfide,5,5'-bis(dimethoxymethylsilylpentyl) trisulfide,3,3'-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide,3,3'-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide.

The preferred sulfur containing organosilicon compounds are the3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. The mostpreferred compound is 3,3'-bis(triethoxysilylpropyl) tetrasulfide.Therefore as to formula I, preferably Z is ##STR6## where R² is analkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularlypreferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3carbon atoms being particularly preferred; and n is an integer of from 3to 5 with 4 being particularly preferred.

The amount of the sulfur containing organosilicon compound in a rubbercomposition will vary depending on the level of silica that is used.Generally speaking, the amount will range from 0.5 to 50 phr.Preferably, the amount will range from 1.5 to 8 phr. Depending on thedesired properties, the weight ratio of the sulfur containingorganosilicon compound to silica may vary. Generally speaking, theweight ratio will range from 1:100 to 1:5. Preferably, the weight ratiowill range from 1:20 to 1:10.

When the sulfur containing organosilicon is present, the rubbercomposition should contain a sufficient amount of silica, and carbonblack, if used, to contribute a reasonably high modulus and highresistance to tear. The silica filler may be added in amounts rangingfrom 10 to 250 phr. Preferably, the silica is present in an amountranging from 15 to 80 phr. If carbon black is also present, the amountof carbon black, if used, may vary. Generally speaking, the amount ofcarbon black will vary from 0 to 80 phr. Preferably, the amount ofcarbon black will range from 0 to 40 phr.

Where the rubber composition contains both silica and carbon black, theweight ratio of silica to carbon black may vary. For example, the weightratio may be as low as 1:5 to a silica to carbon black weight ratio of30:1. Preferably, the weight ratio of silica to carbon black ranges from1:3 to 5:1. The combined weight of the silica and carbon black, ashereinbefore referenced, may be as low as about 30 phr, but ispreferably from about 45 to about 90 phr. It is to be appreciated thatthe sulfur containing organosilicon may be used in conjunction with acarbon black, namely pre-mixed with a carbon black prior to addition tothe rubber composition, and such carbon black is to be included in theaforesaid amount of carbon black for the rubber composition formulation.The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate,e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930).

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300.

The silica might be expected to have an average ultimate particle size,for example, in the range of 0.01 to 0.05 micron as determined by theelectron microscope, although the silica particles may be even smaller,or possibly larger, in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available fromRhone-Poulenc, with, for example, designations of Z1165MP and Z165GR andsilicas available from Degussa AG with, for example, designations VN2and VN3, etc. The PPG Hi-Sil silicas are currently preferred. The rubbercompositions of the present invention may contain a methylene donor anda methylene acceptor. The term "methylene donor" is intended to mean acompound capable of reacting with a methylene acceptor (such asresorcinol or its equivalent containing a present hydroxyl group) andgenerate the resin in-situ. Examples of methylene donors which aresuitable for use in the present invention includehexamethylenetetramine, hexaethoxymethylmelamine,hexamethoxymethylmelamine, lauryloxymethylpyridinium chloride,ethoxymethylpyridinium chloride, trioxan hexamethoxymethylmelamine, thehydroxy groups of which may be esterified or partly esterified, andpolymers of formaldehyde such as paraformaldehyde. In addition, themethylene donors may be N-substituted oxymethylmelamines, of the generalformula: ##STR7## wherein X is an alkyl having from 1 to 8 carbon atoms,R³, R⁴, R⁵, R⁶ and R⁷ are individually selected from the groupconsisting of hydrogen, an alkyl having from 1 to 8 carbon atoms and thegroup --CH₂ OX. Specific methylene donors includehexakis-(methoxymethyl)melamine, N,N',N"-trimethyl/N,N', -N"-trimethylolmelamine hexamethylolmelamine N,N', N"-dimethylolmelamine,N-methylolmelamine, N,N'-dimethylolmelamine,N,N',N"-tris(methoxymethyl)melamine andN,N'N"-tributyl-N,N',N"-trimethylol-melamine. The N-methylol derivativesof melamine are prepared by known methods.

The amount of methylene donor and methylene acceptor that is present inthe rubber stock may vary. Typically, the amount of methylene donor andmethylene acceptor that each is present will range from about 0.1 phr to10.0 phr. Preferably, the amount of methylene donor and methyleneacceptor that each is present ranges from about 2.0 phr to 5.0 phr.

The weight ratio of methylene donor to the methylene acceptor may vary.Generally speaking, the weight ratio will range from about 1:10 to about10:1. Preferably, the weight ratio ranges from about 1:3 to 3:1.

When the compound of the present invention is used as a wire coat orbead coat for use in a tire, the compound generally contains anorgano-cobalt compound which serves as a wire adhesion promoter. Any ofthe organo-cobalt compounds known in the art to promote the adhesion ofrubber to metal may be used. Thus, suitable organo-cobalt compoundswhich may be employed include cobalt salts of fatty acids such asstearic, palmitic, oleic, linoleic and the like; cobalt salts ofaliphatic or alicyclic carboxylic acids having from 6 to 30 carbonatoms; cobalt chloride, cobalt naphthenate; cobalt carboxylate and anorgano-cobalt-boron complex commercially available under the designationManobond C from Wyrough and Loser, Inc, Trenton, N.J. Manobond C isbelieved to have the structure: ##STR8## in which R¹² is an alkyl grouphaving from 9 to 12 carbon atoms.

Amounts of organo-cobalt compound which may be employed depend upon thespecific nature of the organo-cobalt compound selected, particularly theamount of cobalt metal present in the compound. Since the amount ofcobalt metal varies considerably in organo-cobalt compounds which aresuitable for use, it is most appropriate and convenient to base theamount of the organo-cobalt compound utilized on the amount of cobaltmetal desired in the finished stock composition. Accordingly, it may ingeneral be stated that the amount of organo-cobalt compound present inthe stock composition should be sufficient to provide from about 0.01percent to about 0.35 percent by weight of cobalt metal based upon totalweight of the rubber stock composition with the preferred amounts beingfrom about 0.03 percent to about 0.2 percent by weight of cobalt metalbased on total weight of skim stock composition.

The mixing of the rubber composition can be accomplished by methodsknown to those having skill in the rubber mixing art. For example theingredients are typically mixed in at least two stages, namely at leastone non-productive stage followed by a productive mix stage. The finalcuratives including sulfur vulcanizing agents are typically mixed in thefinal stage which is conventionally called the "productive" mix stage inwhich the mixing typically occurs at a temperature, or ultimatetemperature, lower than the mix temperature(s) than the precedingnon-productive mix stage(s). The rubber, silica and sulfur containingorganosilicon, and carbon black if used, may be mixed in one or morenon-productive mix stages. The terms "non-productive" and "productive"mix stages are well known to those having skill in the rubber mixingart.

Upon vulcanization of the sulfur vulcanizable composition at atemperature ranging from 100° C. to 200° C., the rubber composition ofthis invention can be used for various purposes. For example, the sulfurvulcanized rubber composition may be in the form of a tire, belt, hose,motor mounts, gaskets and air springs. In the case of a tire, it can beused for various tire components. Such tires can be built, shaped,molded and cured by various methods which are known and will be readilyapparent to those having skill in such art. Preferably, the rubbercomposition is used in the tread of a tire. As can be appreciated, thetire may be a passenger tire, aircraft tire, truck tire and the like.Preferably, the tire is a passenger tire. The tire may also be a radialor bias, with a radial tire being preferred.

The invention may be better understood by reference to the followingexamples in which the parts and percentages are by weight unlessotherwise indicated.

The following examples are presented in order to illustrate but notlimit the present invention.

Cure properties were determined using a Monsanto oscillating discrheometer which was operated at a temperature of 150° C. and at afrequency of 11 hertz. A description of oscillating disc rheometers canbe found in the Vanderbilt Rubber Handbook edited by Robert O. Ohm(Norwalk, Conn., R. T. Vanderbilt Company, Inc., 1990), pages 554-557.The use of this cure meter and standardized values read from the curveare specified in ASTM D-2084. A typical cure curve obtained on anoscillating disc rheometer is shown on page 555 of the 1990 edition ofthe Vanderbilt Rubber Handbook.

In such an oscillating disc rheometer, compounded rubber samples aresubjected to an oscillating shearing action of constant amplitude. Thetorque of the oscillating disc embedded in the stock that is beingtested that is required to oscillate the rotor at the vulcanizationtemperature is measured. The values obtained using this cure test arevery significant since changes in the rubber or the compounding recipeare very readily detected. It is obvious that it is normallyadvantageous to have a fast cure rate.

The formulation set out in Table 1 was utilized for all the examplesunless otherwise stated. The various additives were compounded usingconventional rubber compounding techniques and the samples vulcanized bycompression molding methods for 36 minutes at 150° C. unless otherwisestated.

                  TABLE 1                                                         ______________________________________                                        Non-Productive                                                                Antioxidant.sup.1 1.00                                                        Polyisoprene      50.00                                                       SBR.sup.2 1712C   69.75                                                       Processing Oil    10.00                                                       Stearic acid      2.00                                                        Zinc Oxide        3.00                                                        Carbon Black.sup.3                                                                              50.00                                                       Productive                                                                    Sulfur            1.75                                                        Sulfenamide.sup.4 1.25                                                        Hydrated Thiosulfate                                                                            variable                                                    ______________________________________                                         .sup.1 diarylphenylenediamine                                                 .sup.2 emulsion polymerized styrenebutadiene rubber available from The        Goodyear Tire & Rubber Co under the designation SBR 1712C                     .sup.3 N299                                                                   .sup.4 Ncyclohexyl-2-benzothiazolesulfenamide                            

EXAMPLE 1

In this example, sodium thiosulfate pentahydrate is evaluated as a cureactivator which can be used to beneficially reduce cure times withoutsacrificing cured physical properties of the resultant vulcanizate. Therubber compositions are identified herein as Samples A, B, C, D and E ofTable 2 with Sample A acting as the control compound containing nosodium thiosulfate pentahydrate, and Samples B, C, D and E utilizingsodium thiosulfate pentahydrate varying amounts from 0.5 phr to 5.0 phr,respectively. The date illustrates that with the addition of sodiumthiosulfate pentahydrate to control A cure times were substantiallyreduced (Sample B with 0.5 phr sodium thiosulfate pentahydrate gave acure time reduction of 27.9 percent; Sample C, a 41.0 percent reductionwith 1.0 phr sodium thiosulfate pentahydrate; Sample D, a 55.7 percentreduction with 2.0 phr sodium thiosulfate pentahydrate; and Sample E, a60.7 percent reduction with 5.0 phr sodium thiosulfate pentahydrate)without significantly impacting the physical properties of the finalvulcanizate.

                                      TABLE 2                                     __________________________________________________________________________    Sample #         A   B   C   D   E                                            __________________________________________________________________________    Sodium Thiosulfate Pentahydrate                                                                none                                                                              0.5 1.0 2.0 5.0                                          Monsanto Rheometer 1° Arc,                                             150° C.                                                                M.sub.HF Torque Units (dNm)                                                                    33  32.5                                                                              32.3                                                                              32.5                                                                              32                                           M.sub.L Torque Units (dNm)                                                                     8.0 8.5 8.0 8.0 7.5                                          M.sub.HF --M.sub.L Torque Units (dNm)                                                          25.0                                                                              24.3                                                                              24.3                                                                              24.5                                                                              24.5                                         Cure Time, t'c(25), min                                                                        20.0                                                                              12.2                                                                              8.3 4.5 3.6                                          Cure Time, t'c(90), min                                                                        30.5                                                                              22.0                                                                              18  13.5                                                                              12.0                                         % Reduction in t'c(90) cure time                                                                   27.9                                                                              41.0                                                                              55.7                                                                              60.7                                         Stress-Strain Data                                                            Modulus at 300% Elongation, MPa                                                                6.99                                                                              7.19                                                                              7.35                                                                              7.45                                                                              7.35                                         Tensile Strength, MPa                                                                          18.38                                                                             18.56                                                                             18.9                                                                              17.27                                                                             18.38                                        Elongation at Break, %                                                                         605 602 602 567 594                                          Shore A Hardness at 100° C.                                                             48.4                                                                              48.6                                                                              48.4                                                                              49.6                                                                              48.3                                         Percent Rebound at 100° C.                                                              59  60  59.4                                                                              60.4                                                                              59.8                                         __________________________________________________________________________

EXAMPLE II

In this example, sodium thiosulfate pentahydrate, which can be used tobeneficially reduce cure times, is compared to anhydrous sodiumthiosulfate as a cure activator. The rubber compositions are identifiedherein as Samples F, G and H of Table 3 with Sample H acting as thecontrol compound containing no sodium thiosulfate pentahydrate, Sample Fcontaining sodium thiosulfate pentahydrate, and Sample G containing anequal molar equivalent of anhydrous sodium thiosulfate for comparisonversus Sample F. The data unexpectively shows that with anhydrous sodiumthiosulfate (Sample G) cure times were not reduced when compared to thecontrol (Sample H) whereas with sodium thiosulfate pentahydrate (SampleF) cure times were substantially reduced when compared to the control.This illustrates the unique and unobvious character of the hydrated saltof sodium thiosulfate.

    ______________________________________                                        Sample                  F      G      H                                       ______________________________________                                        Sodium Thiosulfate Pentahydrate (4.0 mmols)                                                           1.0    0      0                                       Sodium thiosulfate (anhydrous) (4.0 mmols)                                                            0      0.64   0                                       Monsanto Rheometer 1° Arc, 150° C.                              M.sub.HF Torque Unite (dNm)                                                                           32     32.5   31.5                                    M.sub.L Torque Unites (dNm)                                                                           8      7.5    7.3                                     M.sub.HF --M.sub.L Torque Units (dNm)                                                                 24     25     24.2                                    Cure Time, t'c(25), min 8.5    21     19.7                                    Cure Time, t'c(90), min,                                                                              17.5   31.8   29.5                                    % Reduction in t'c(90) cure time                                                                      40.7   none                                           ______________________________________                                    

EXAMPLE III

In this example, sodium thiosulfate pentahydrate is evaluated as a cureactivator for a variety of sulfenamide-type accelerators. The rubbercompositions are identified herein as Samples I, J, K, L, M, N, O, P, Qand R of Table 4 with Sample I, K, M, O and Q acting as the controlcompounds containing no sodium thiosulfate pentahydrate, and Samples J,L, N, P and R contain sodium thiosulfate pentahydrate at 0.50 phr. Table4 illustrates the cure activating power of sodium thiosulfatepentahydrate when used in conjunction with sulfenamide-typeaccelerators. Cure time reductions of 29.5 percent, 33.3 percent, 14.2percent and 27.5 percent were respectively obtained when 0.5 phr ofsodium thiosulfate pentahydrate was added to the formulations containingCBS (Sample J versus Sample I), TBBS (Sample L versus Sample K), DCBS(Sample N versus Sample M) and TBSI (Sample P versus Sample O). The useof sodium thiosulfate pentahydrate with MBTS did not provide anyreduction in t'c(90) cure time. This shows that to be useful in reducingcure times, a sulfenamide accelerator should be present as part of thecure system.

                                      TABLE 4                                     __________________________________________________________________________                                                    Control                       Sample            I  J  K   L   M   N   O   P   Q   R                         __________________________________________________________________________    Accelerator       CBS                                                                              CBS                                                                              TBBS                                                                              TBBS                                                                              DCBS                                                                              DCBS                                                                              TBSI                                                                              TBSI                                                                              MBTS                                                                              MBTS                      (1.25 phr)                                                                    Sodium            0  0.50                                                                             0   0.50                                                                              0   0.50                                                                              0   0.50                                                                              0   0.50                      Thiosulfate                                                                   Pentahydrate                                                                  Monsanto Rheometer 1° Arc, 150° C.                              M.sub.HF Torque   30.0                                                                             30.0                                                                             31.0                                                                              30.5                                                                              27.0                                                                              27.0                                                                              31.0                                                                              31.5                                                                              27.0                                                                              26.2                      Units (dNm)                                                                   M.sub.L Torque    8.0                                                                              7.5                                                                              7.0 7.0 7.0 7.5 7.0 8.0 6.0 6.0                       Units (dNm)                                                                   M.sub.HF --M.sub.L Torque                                                                       22.0                                                                             22.5                                                                             24.0                                                                              23.5                                                                              20.0                                                                              19.5                                                                              24.0                                                                              23.5                                                                              21.0                                                                              20.2                      Units (dNm)                                                                   Cure Time,        14.0                                                                             8.0                                                                              17.0                                                                              9.5 23.5                                                                              19.0                                                                              22.0                                                                              13.0                                                                              9.5 6.0                       t'c(25), min                                                                  Cure Time,        22.0                                                                             15.5                                                                             25.5                                                                              17.0                                                                              42.0                                                                              36.0                                                                              34.5                                                                              25.0                                                                              34.0                                                                              34.0                      t'c(90), min                                                                  % Reduction in       29.5   33.3    14.2    27.5    none                      t'c(90)                                                                       cure time                                                                     Stress-Strain Data                                                            Modulus at        6.86                                                                             6.77                                                                             7.39                                                                              7.3 4.84                                                                              5.16                                                                              6.75                                                                              7.13                                                                              5.46                                                                              5.12                      300%                                                                          Elongation, MPa                                                               Tensile Strength, 19.05                                                                            18.99                                                                            19.51                                                                             19.07                                                                             19.41                                                                             19.66                                                                             19.27                                                                             20.4                                                                              19.8                                                                              19.37                     MPa                                                                           Elongation at     614                                                                              612                                                                              600 593 744 727 622 629 708 721                       Break, %                                                                      Shore A           47.7                                                                             47.4                                                                             49  48.7                                                                              43.4                                                                              43.9                                                                              47.5                                                                              48.2                                                                              43.6                                                                              42.6                      Hardness at       60.8                                                                             61.3                                                     100° C.                                                                Percent Rebound   60.4                                                                             60.9                                                                             61.3                                                                              60.9                                                                              56.8                                                                              56.2                                                                              60.8                                                                              61.3                                                                              58.1                                                                              56                        at 100° C.                                                             __________________________________________________________________________     CBS = Ncyclohexyl-2-benzothiazolesulfenamide                                  TBBS = Ntert-butyl-2-benzothiazolesulfenamide                                 DCBS = N,Ndicyclohexyl-2-benzothiazolesulfenamide                             TBSI = Ntert-butyl bis2(2-benzothiazolesulfen)amide, Santocure ® TBSI     MBTS = 2,2dithiobisbenzothiazole, (Altax)                                

EXAMPLE IV

In this example, the vulcanizing activity of sodium thiosulfatepentahydrate is evaluated. Sample V illustrates sodium thiosulfatepentahydrate's vulcanizing activity in the sulfur vulcanizable rubber ofTable 1 when no sulfur or sulfenamide accelerator such as CBS ispresent. As can be seen by the data, no cure takes place in the absentof sulfur and sulfenamide accelerator. Likewise as Sample W illustrates,no useful vulcanizate can be obtained when sodium thiosulfatepentahydrate and a sulfenamide accelerator such as CBS is cured in theabsence of sulfur. When sulfur is added to the sulfur vulcanizablerubber composition of Sample W to produce Sample Y, a useful cure timereduction of 17.5 minutes is noted. Also, useful mechanical propertiesfor the vulcanizate are obtained when sulfur is added to the sulfurvulcanizable rubber composition of Sample W providing large improvementsin modulus at 300 percent elongation, tensile strength, elongation atbreak, hardness and percent rebound.

                  TABLE 5                                                         ______________________________________                                        Sample          V        W       X     Y                                      ______________________________________                                        CBS.sup.1                1.25    1.25  1.25                                   sodium thiosulfate                                                                            0.5      0.5           0.5                                    pentahydrate                                                                  Sulfur                           1.75  1.75                                   Monsanto Rheometer 1°                                                  Arc, 150° C.                                                           M.sub.HF Torque Units (dNm)                                                                   No Cure  10      30    30                                     M.sub.L Torque Units (dNm)                                                                    No Cure  5       4.5   5                                      M.sub.HF --M.sub.L Torque Units                                                               No Cure  5       25.5  25                                     (dNm)                                                                         Cure Time, t'c(25), min                                                                       No Cure  12      15    8.2                                    Cure Time, t'c(90), min                                                                       No Cure  35      22    17.5                                   Stress-Strain Data                                                            Modulus at 300% Elonga-  0.8     7.55  7.19                                   tion, MPa                                                                     Tensile Strength, MPa                                                                         0.34     4.87    17.88 18.62                                  Elongation at Break, %                                                                        284      1062    561   589                                    Shore A Hardness at 100° C.                                                            5.8      16.8    48.4  47.4                                   Percent Rebound at 100° C.                                                             31.4     34.6    61.3  60.8                                   ______________________________________                                         .sup.1 Ncyclohexyl-2-benzothiazesulfenamide                              

EXAMPLE V

In this example, other hydrated salts of thiosulfate are evaluated forcure activating potential. The rubber compositions are identified hereinas Samples Z, AB, AC and AD of Table 6 with Sample Z acting as thecontrol compound containing no hydrated salt of thiosulfate, Sample ABcontaining 4.0 mmoles of potassium thiosulfate hydrate (1.5 moles ofwater), Sample AC containing 4.0 mmols of magnesium thiosulfatehexahydrate and Sample AD containing 4.0 mmols of sodium thiosulfatepentahydrate. In each example, cure time reductions are observed whencompared the control with sodium thiosulfate pentahydrate giving thegreatest reduction in cure times.

                  TABLE 6                                                         ______________________________________                                        Samples          Z       AB      AC    AD                                     ______________________________________                                        potassium thiosulfate hydrate.sup.1                                                                    0.88                                                 4.0 mmols (phr)                                                               magnesium thiosulfate hexa-      1                                            hydrate 4.0 mmols (phr)                                                       sodium thiosulfate pentahydrate        1                                      4.0 mmols (phr)                                                               Monsanto Rheometer 1° Arc,                                             150° C.                                                                M.sub.HF Torque Units (dNm)                                                                    30      31      28.5  29.5                                   M.sub.L Torque Units (dNm)                                                                     4.5     5       5     5                                      M.sub.HF --M.sub.L Torque                                                                      25.5    26      23.5  24.5                                   Units (dNm)                                                                   Cure Time, t'c(25), min                                                                        15      9.5     5.2   4.5                                    Cure Time, t'c(90), min                                                                        22      17      15    12                                     % Reduction in t'(90) cure                                                                             22.7    31.9  45.5                                   time                                                                          Stress-Strain Data                                                            Modulus at 300% Elongation,                                                                    7.55    7.51    6.51  7.26                                   MPa                                                                           Tensile Strength, MPa                                                                          17.88   16.83   17.44 19.0                                   Elongation at Break, %                                                                         561     543     604   603                                    ______________________________________                                         .sup.1 K.sub.2 S.sub.2 O.sub.3.1.5 H.sub.2 O                             

What is claimed is:
 1. A method for increasing the rate of vulcanizationof a sulfur vulcanizable rubber composition by heating a sulfurvulcanizable composition to a temperature ranging from 100° C. to 200°C., said rubber composition comprising(a) a sulfur vulcanizable rubber(b) from 0.5 phr to 5 phr of a sulfenamide compound of the generalformula: ##STR9## wherein R¹ is hydrogen, an acyclic aliphatic grouphaving from about 1 to 10 carbon atoms, or a cyclic aliphatic grouphaving from about 5 to 10 carbon atoms; and R² is hydrogen, a cyclicaliphatic group having from 5 to 10 carbon atoms or amercaptobenzothiazolyl group of the formula: ##STR10## (c) from 0.05 to10 phr of a hydrated thiosulfate.
 2. The method of claim 1 wherein saidhydrated thiosulfate is selected from the group consisting of BaS₂ O₃.H₂O, K₂ S₂ O₃.1.5 H₂ O, CaS₂ O₃.6H₂ O, MgS₂ O₃.6H₂ O, NiS₂ O₃.6H₂ O, CoS₂O₃.6H₂ O, SrS₂ O₃.5H₂ O, Na₂ S₂ O₃.5H₂ O, MnS₂ O₃.5H₂ O, Li₂ S₂ O₃.3H₂ Oand CdS₂ O₃.5H₂ O.
 3. The method of claim 2 wherein said hydratedthiosulfate is Na₂ S₂ O₃.5H₂ O.
 4. The method of claim 3 wherein saidNa₂ S₂ O₃.5H₂ O is present in an amount ranging from 0.10 to 5.0 phr. 5.The method of claim 1 wherein a sulfur vulcanizing agent is present insaid sulfur vulcanizable rubber composition and is selected from thegroup consisting of elemental sulfur, an amine disulfide, polymericpolysulfide and sulfur olefin adducts.
 6. The method of claim 5 whereinsaid sulfur vulcanizing agent is present in an amount ranging from 0.5to 8 phr.
 7. The method of claim 1 wherein said sulfenamide compound isselected from the group consisting ofN-cyclohexyl-2-benzothiazylsulfenamide,N-isopropyl-2-benzothiazylsulfenamide,N-t-butyl-2-benzothiazyl-sulfenamide, N-t-butylbis-(2-benzothiazylsulfen)amide andN,N-dicyclohexyl-2-benzothiazylsulfenamide.
 8. The method of claim 5said sulfenamide compound is added to said vulcanizable rubbercomposition in amounts ranging from 0.7 to 2 phr.
 9. The method of claim1 wherein said sulfur vulcanizable rubber is an elastomer containingolefinic unsaturation and is selected from the group consisting ofnatural rubber, neoprene, polyisoprene, butyl rubber, polybutadiene,styrene-butadiene copolymer, styrene-isoprene copolymer,styrene-isoprene-butadiene terpolymer, methyl methacrylate-butadienecopolymer, isoprene-styrene copolymer, methyl methacrylate-isoprenecopolymer, acrylonitrile-isoprene copolymer, acrylonitrile-butadienecopolymer, ethylene-propylene-diene monomer rubber and mixtures thereof.10. A sulfur vulcanizable rubber composition comprising(a) a sulfurvulcanizable rubber (b) from 0.5 phr to 5 phr of a sulfenamide compoundof the general formula: ##STR11## wherein R¹ is hydrogen, an acyclicaliphatic group having from about 1 to 10 carbon atoms, or a cyclicaliphatic group having from about 5 to 10 carbon atoms; and R² ishydrogen, a cyclic aliphatic group having from 5 to 10 carbon atoms or amercaptobenzothiazolyl group of the formula: ##STR12## (c) from 0.05 to10 phr of a hydrated thiosulfate.
 11. The composition of claim 10wherein said hydrated thiosulfate is selected from the group consistingof BaS₂ O₃.H₂ O, K₂ S₂ O₃.1.5 H₂ O, CaS₂ O₃.6H₂ O, MgS₂ O₃.6H₂ O, NiS₂O₃.6H₂ O, CoS₂ O₃.6H₂ O, SrS₂ O₃.5H₂ O, Na₂ S₂ O₃.5H₂ O, MnS₂ O₃.5H₂ O,Li₂ S₂ O₃.3H₂ O and CdS₂ O₃.5H₂ O.
 12. The composition of claim 11wherein said hydrated thiosulfate is Na₂ S₂ O₃.5H₂ O.
 13. Thecomposition of claim 12 wherein said Na₂ S₂ O₃.5H₂ O is present in anamount ranging from 0.10 to 5.0 phr.
 14. The composition of claim 10wherein a sulfur vulcanizing agent is present in said sulfurvulcanizable rubber composition and is selected from the groupconsisting of elemental sulfur, an amine disulfide, polymericpolysulfide and sulfur olefin adducts.
 15. The composition of claim 14wherein said sulfur vulcanizing agent is present in an amount rangingfrom 0.5 to 8 phr.
 16. The composition of claim 10 wherein saidsulfenamide is present in an amount ranging from 0.7 to 2 phr.
 17. Thecomposition of claim 10 wherein said sulfur vulcanizable rubber is anelastomer containing olefinic unsaturation and is selected from thegroup consisting of natural rubber, neoprene, polyisoprene, butylrubber, polybutadiene, styrene-butadiene copolymer, styrene-isoprenecopolymer, styrene-isoprene-butadiene terpolymer, methylmethacrylate-butadiene copolymer, isoprene-styrene copolymer, methylmethacrylate-isoprene copolymer, acrylonitrile-isoprene copolymer,acrylonitrile-butadiene copolymer, ethylene-propylene-diene monomerrubber and mixtures thereof.
 18. A sulfur vulcanized rubber compositionwhich is prepared by heating the sulfur vulcanizable rubber compositionof claim 10 to a temperature ranging from 100° C. to 200° C.